Post rip image rendering in an electrographic printer using density patch feedback

ABSTRACT

An electrostatographic recording method for printing an image on a receiver comprising the steps of: operating a primary charger to establish a uniform primary voltage level on an image recording member; developing a control patch on the image recording member, measuring density of the control patch to thereby provide a density measurement signal; converting the image into a digital bitmap comprised of an array of pixels wherein each pixel is assigned a digital value representing marking information; rendering the digital bitmap by defining each pixel as either a background pixel, interior pixel, or an edge pixel and reassigning the digital value of one or more of edge pixels or interior pixels independently and as a function of the density measurement signal; modulating electrostatic charge on the image recording member as a function of the digital bitmap after rendering.

RELATED APPLICATIONS

[0001] This application claims the priority date of U.S. ProvisionalApplication serial No. 60/459,119 filed Mar. 31, 2003 entitled “POST RIPIMAGE RENDERING IN AN ELECTROGRAPHIC PRINTER USING DENSITY PATCHFEEDBACK”.

FIELD OF THE INVENTION

[0002] This invention is in the field of digital printing, and is morespecifically directed to image exposure control in electrostatographicprinters.

BACKGROUND OF THE INVENTION

[0003] Electrographic printing has become the prevalent technology formodem computer-driven printing of text and images, on a wide variety ofhard copy media. This technology is also referred to as electrographicmarking, electrostatographic printing or marking, andelectrophotographic printing or marking. Conventional electrographicprinters are well suited for high resolution and high speed printing,with resolutions of 600 dpi (dots per inch) and higher becomingavailable even at modest prices. As will be described below, at theseresolutions, modem electrographic printers and copiers are well-suitedto be digitally controlled and driven, and are thus highly compatiblewith computer graphics and imaging. Controlling the appearance ofprinted images is an important aspect of printers. An example of suchcontrol efforts is described in U.S. Pat. No. 6,181,438, which is herebyincorporated herein by reference.

[0004] Efforts regarding printers or printing systems have led tocontinuing developments to improve their versatility practicality, andefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIGS. 1a-1 b are schematic diagrams of an electrographic markingor reproduction system in accordance with the present invention.

[0006]FIG. 2 is a schematic block diagram for image rendering inaccordance with the present invention.

[0007]FIG. 3 is a flow chart for image rendering in accordance with thepresent invention.

[0008]FIG. 4 is a schematic diagram of designations for a directionallook up table in accordance with the present invention.

[0009]FIGS. 5a-5 d are representations of a pixel grid having a tonedimage provided thereon in accordance with the present invention.

[0010]FIGS. 6a-6 f are representations of a pixel grid having an imageprovided thereon in accordance with the present invention.

[0011]FIG. 7a is a representation of a pixel grid having a one pixelwide toned image provided thereon in accordance with the presentinvention.

[0012]FIG. 7b is a representation of a pixel grid with edge pixeldesignations for the toned image of FIG. 7a in accordance with thepresent invention.

[0013]FIG. 7c is a representation of a pixel grid with direction valuesfor the toned image of FIG. 7a in accordance with the present invention.

[0014]FIG. 7d is a representation of a pixel grid with background pixel,edge pixel and one pixel wide line assignment values for the toned imageof FIG. 7a in accordance with the present invention.

[0015]FIG. 8a is a representation of a pixel grid having a two pixelwide toned image provided thereon in accordance with the presentinvention.

[0016]FIG. 8b is a representation of a pixel grid with edge pixelassignments for the toned image of FIG. 8a in accordance with thepresent invention.

[0017]FIG. 8c is a representation of a pixel grid with direction valuesfor the toned image of FIG. 8a in accordance with the present invention.

[0018]FIG. 8d is a representation of a pixel grid with background pixel,edge pixel and two pixel wide line assignment values for the toned imageof FIG. 8a in accordance with the present invention.

[0019]FIG. 9 is a schematic representation of an exemplary adjustmentinterface for assigning new pixel values according to the presentinvention.

[0020]FIG. 10 is a representation of a pixel grid with alternative pixelassignments in accordance with the present invention for a letter O.

[0021]FIG. 11 is an example of a tone reproduction curve for anelectrographic printer in accordance with the present invention.

[0022]FIG. 12 is a copy of a series of printed halftone steps for threedifferent screen frequencies.

[0023]FIG. 13 is a graph illustrating percent lightness vs percent blackpixels for each step for each screen frequency shown in FIG. 14.

[0024]FIG. 14 is a copy of a series of printed lines that are 1, 2, 3,4, and 8 pixels wide.

[0025]FIG. 15 is a graph illustrating linewidth vs the number of pixelscounted across the line for an exemplary series of lines of FIG. 14.

[0026]FIG. 16 is a graph illustrating best fit lines extracted forlinewidth vs the number of pixels derived by selecting a fixed IPV andvarying EPV, 2PV and 1PV for eight different cases.

[0027]FIG. 17 is a flow chart illustrating the steps taken to thin anobject by more than one pixel, in accordance with the present invention.

DETAILED DESCRIPTION

[0028] Referring to FIG. 1, a printer machine 10 includes a movingrecording member such as a photoconductive belt 18 which is entrainedabout a plurality of rollers or other supports 21 a through 21 g, one ormore of which is driven by a motor to advance the belt. By way ofexample, roller 21 a is illustrated as being driven by motor 20. Motor20 preferably advances the belt at a high speed, such as 20 inches persecond or higher, in the direction indicated by arrow P, past a seriesof workstations of the printer machine 10. Alternatively, belt 18 may bewrapped and secured about only a single drum.

[0029] Printer machine 10 includes a controller or logic and controlunit (LCU) 24, preferably a digital computer or microprocessor operatingaccording to a stored program for sequentially actuating theworkstations within printer machine 10, effecting overall control ofprinter machine 10 and its various subsystems. LCU 24 also is programmedto provide closed-loop control of printer machine 10 in response tosignals from various sensors and encoders. Aspects of process controlare described in U.S. Pat. No. 6,121,986 incorporated herein by thisreference.

[0030] A primary charging station 28 in printer machine 10 sensitizesbelt 18 by applying a uniform electrostatic corona charge, fromhigh-voltage charging wires at a predetermined primary voltage, to asurface 18 a of belt 18. The output of charging station 28 is regulatedby a programmable voltage controller 30, which is in turn controlled byLCU 24 to adjust this primary voltage, for example by controlling theelectrical potential of a grid and thus controlling movement of thecorona charge. Other forms of chargers, including brush or rollerchargers, may also be used.

[0031] An exposure station 34 in printer machine 10 projects light froma writer 34 a to belt 18. This light selectively dissipates theelectrostatic charge on photoconductive belt 18 to form a latentelectrostatic image of the document to be copied or printed. Writer 34 ais preferably constructed as an array of light emitting diodes (LEDs),or alternatively as another light source such as a laser or spatiallight modulator. Writer 34 a exposes individual picture elements(pixels) of belt 18 with light at a regulated intensity and exposure, inthe manner described below. The exposing light discharges selected pixellocations of the photoconductor, so that the pattern of localizedvoltages across the photoconductor corresponds to the image to beprinted. An image is a pattern of physical light which may includecharacters, words, text, and other features such as graphics, photos,etc. An image may be included in a set of one or more images, such as inimages of the pages of a document. An image may be divided intosegments, objects, or structures each of which is itself an image. Asegment, object or structure of an image may be of any size up to andincluding the whole image.

[0032] Image data to be printed is provided by an image data source 36,which is a device that can provide digital data defining a version ofthe image. Such types of devices are numerous and include computer ormicrocontroller, computer workstation, scanner, digital camera, etc.These data represent the location and intensity of each pixel that isexposed by the printer. Signals from data source 36, in combination withcontrol signals from LCU 24 are provided to a raster image processor(RIP) 37. The Digital images (including styled text) are converted bythe RIP 37 from their form in a page description language (PDL) languageto a sequence of serial instructions for the electrographic printer in aprocess commonly known as “ripping” and which provides a ripped image toa image storage and retrieval system known as a Marking Image Processor(MIP) 38.

[0033] In general, the major roles of the RIP 37 are to: receive jobinformation from the server; Parse the header from the print job anddetermine the printing and finishing requirements of the job; Analyzethe PDL (Page Description Language) to reflect any job or pagerequirements that were not stated in the header; Resolve any conflictsbetween the requirements of the job and the Marking Engine configuration(i.e., RIP time mismatch resolution); Keep accounting record and errorlogs and provide this information to any subsystem, upon request;Communicate image transfer requirements to the Marking Engine; Translatethe data from PDL (Page Description Language) to Raster for printing;and Support Diagnostics communication between User Applications The RIPaccepts a print job in the form of a Page Description Language (PDL)such as PostScript, PDF or PCL and converts it into Raster, a form thatthe marking engine can accept. The PDL file received at the RIPdescribes the layout of the document as it was created on the hostcomputer used by the customer. This conversion process is calledrasterization. The RIP makes the decision on how to process the documentbased on what PDL the document is described in. It reaches this decisionby looking at the first 2K of the document. A job manager sends the jobinformation to a MSS (Marking Subsystem Services) via Ethernet and therest of the document further into the RIP to get rasterized. Forclarification, the document header contains printer-specific informationsuch as whether to staple or duplex the job. Once the document has beenconverted to raster by one of the interpreters, the Raster data goes tothe MIP 38 via RTS (Raster Transfer Services); this transfers the dataover a IDB (Image Data Bus).

[0034] The MIP functionally replaces recirculating feeders on opticalcopiers. This means that images are not mechanically rescanned withinjobs that require rescanning, but rather, images are electronicallyretrieved from the MIP to replace the rescan process. The MIP acceptsdigital image input and stores it for a limited time so it can beretrieved and printed to complete the job as needed. The MIP consists ofmemory for storing digital image input received from the RIP. Once theimages are in MIP memory, they can be repeatedly read from memory andoutput to the Render Circuit. The amount of memory required to store agiven number of images can be reduced by compressing the images;therefore, the images are compressed prior to MIP memory storage, thendecompressed while being read from MIP memory.

[0035] The output of the MIP is provided to an image render circuit 39,which alters the image and provides the altered image to the writerinterface 32 (otherwise known as a write head, print head, etc.) whichapplies exposure parameters to the exposure medium, such as aphotoconductor 18.

[0036] After exposure, the portion of exposure medium belt 18 bearingthe latent charge images travels to a development station 35.Development station 35 includes a magnetic brush in juxtaposition to thebelt 18. Magnetic brush development stations are well known in the art,and are preferred in many applications; alternatively, other known typesof development stations or devices may be used. Plural developmentstations 35 may be provided for developing images in plural colors, orfrom toners of different physical characteristics. Full process colorelectrographic printing is accomplished by utilizing this process foreach of four toner colors (e.g., black, cyan, magenta, yellow).

[0037] Upon the imaged portion of belt 18 reaching development station35, LCU 24 selectively activates development station 35 to apply tonerto belt 18 by moving backup roller 35 a belt 18, into engagement with orclose proximity to the magnetic brush. Alternatively, the magnetic brushmay be moved toward belt 18 to selectively engage belt 18. In eithercase, charged toner particles on the magnetic brush are selectivelyattracted to the latent image patterns present on belt 18, developingthose image patterns. As the exposed photoconductor passes thedeveloping station, toner is attracted to pixel locations of thephotoconductor and as a result, a pattern of toner corresponding to theimage to be printed appears on the photoconductor. As known in the art,conductor portions of development station 35, such as conductiveapplicator cylinders, are biased to act as electrodes. The electrodesare connected to a variable supply voltage, which is regulated byprogrammable controller 40 in response to LCU 24, by way of which thedevelopment process is controlled.

[0038] Development station 35 may contain a two component developer mixwhich comprises a dry mixture of toner and carrier particles. Typicallythe carrier preferably comprises high coercivity (hard magnetic) ferriteparticles. As an example, the carrier particles have a volume-weighteddiameter of approximately 30μ. The dry toner particles are substantiallysmaller, on the order of 6μ to 15μ in volume-weighted diameter.Development station 35 may include an applicator having a rotatablemagnetic core within a shell, which also may be rotatably driven by amotor or other suitable driving means. Relative rotation of the core andshell moves the developer through a development zone in the presence ofan electrical field. In the course of development, the toner selectivelyelectrostatically adheres to photoconductive belt 18 to develop theelectrostatic images thereon and the carrier material remains atdevelopment station 35. As toner is depleted from the developmentstation due to the development of the electrostatic image, additionaltoner is periodically introduced by toner auger 42 into developmentstation 35 to be mixed with the carrier particles to maintain a uniformamount of development mixture. This development mixture is controlled inaccordance with various development control processes. Single componentdeveloper stations, as well as conventional liquid toner developmentstations, may also be used.

[0039] A transfer station 46 in printing machine 10 moves a receiversheet S into engagement with photoconductive belt 18, in registrationwith a developed image to transfer the developed image to receiver sheetS. Receiver sheets S may be plain or coated paper, plastic, or anothermedium capable of being handled by printer machine 10. Typically,transfer station 46 includes a charging device for electrostaticallybiasing movement of the toner particles from belt 18 to receiver sheetS. In this example, the biasing device is roller 46 b, which engages theback of sheet S and which is connected to programmable voltagecontroller 46 a that operates in a constant current mode duringtransfer. Alternatively, an intermediate member may have the imagetransferred to it and the image may then be transferred to receiversheet S. After transfer of the toner image to receiver sheet S, sheet Sis detacked from belt 18 and transported to fuser station 49 where theimage is fixed onto sheet S, typically by the application of heat.Alternatively, the image may be fixed to sheet S at the time oftransfer.

[0040] A cleaning station 48, such as a brush, blade, or web is alsolocated behind transfer station 46, and removes residual toner from belt18. A pre-clean charger (not shown) may be located before or at cleaningstation 48 to assist in this cleaning. After cleaning, this portion ofbelt 18 is then ready for recharging and re-exposure. Of course, otherportions of belt 18 are simultaneously located at the variousworkstations of printing machine 10, so that the printing process iscarried out in a substantially continuous manner.

[0041] LCU 24 provides overall control of the apparatus and its varioussubsystems as is well known. LCU 24 will typically include temporarydata storage memory, a central processing unit, timing and cycle controlunit, and stored program control. Data input and output is performedsequentially through or under program control. Input data can be appliedthrough input signal buffers to an input data processor, or through aninterrupt signal processor, and include input signals from variousswitches, sensors, and analog-to-digital converters internal to printingmachine 10, or received from sources external to printing machine 10,such from as a human user or a network control. The output data andcontrol signals from LCU 24 are applied directly or through storagelatches to suitable output drivers and in turn to the appropriatesubsystems within printing machine 10.

[0042] Process control strategies generally utilize various sensors toprovide real-time closed-loop control of the electrostatographic processso that printing machine 10 generates “constant” image quality output,from the user's perspective. Real-time process control is necessary inelectrographic printing, to account for changes in the environmentalambient of the photographic printer, and for changes in the operatingconditions of the printer that occur over time during operation(rest/run effects). An important environmental condition parameterrequiring process control is relative humidity, because changes inrelative humidity affect the charge-to-mass ratio Q/m of tonerparticles. The ratio Q/m directly determines the density of toner thatadheres to the photoconductor during development, and thus directlyaffects the density of the resulting image. System changes that canoccur over time include changes due to aging of the printhead (exposurestation), changes in the concentration of magnetic carrier particles inthe toner as the toner is depleted through use, changes in themechanical position of primary charger elements, aging of thephotoconductor, variability in the manufacture of electrical componentsand of the photoconductor, change in conditions as the printer warms upafter power-on, triboelectric charging of the toner, and other changesin electrographic process conditions. Because of these effects and thehigh resolution of modem electrographic printing, the process controltechniques have become quite complex.

[0043] Process control sensor may be a densitometer 76, which monitorstest patches that are exposed and developed in non-image areas ofphotoconductive belt 18 under the control of LCU 24. Densitometer 76 mayinclude a infrared or visible light LED, which either shines through thebelt or is reflected by the belt onto a photodiode in densitometer 76.These toned test patches are exposed to varying toner density levels,including full density and various intermediate densities, so that theactual density of toner in the patch can be compared with the desireddensity of toner as indicated by the various control voltages andsignals. These densitometer measurements are used to control primarycharging voltage V_(O), maximum exposure light intensity E_(O), anddevelopment station electrode bias V_(B). In addition, the processcontrol of a toner replenishment control signal value or a tonerconcentration setpoint value to maintain the charge-to-mass ratio Q/m ata level that avoids dusting or hollow character formation due to lowtoner charge, and also avoids breakdown and transfer mottle due to hightoner charge for improved accuracy in the process control of printingmachine 10. The toned test patches are formed in the interframe area ofbelt 18 so that the process control can be carried out in real timewithout reducing the printed output throughput. Another sensor usefulfor monitoring process parameters in printer machine 10 is electrometerprobe 50, mounted downstream of the corona charging station 28 relativeto direction P of the movement of belt 18. An example of an electrometeris described in U.S. Pat. No. 5,956,544 incorporated herein by thisreference.

[0044] Other approaches to electrographic printing process control maybe utilized, such as those described in International Publication NumberWO 02/10860 A1, and International Publication Number WO 02/14957 A1,both commonly assigned herewith and incorporated herein by thisreference.

[0045] Raster image processing begins with a page description generatedby the computer application used to produce the desired image. TheRaster Image Processor interprets this page description into a displaylist of objects. This display list contains a descriptor for each textand non-text object to be printed; in the case of text, the descriptorspecifies each text character, its font, and its location on the page.For example, the contents of a word processing document with styled textis translated by the RIP into serial printer instructions that include,for the example of a binary black printer, a bit for each pixel locationindicating whether that pixel is to be black or white. Binary printmeans an image is converted to a digital array of pixels, each pixelhaving a value assigned to it, and wherein the digital value of everypixel is represented by only two possible numbers, either a one or azero. The digital image in such a case is known as a binary image.Multi-bit images, alternatively, are represented by a digital array ofpixels, wherein the pixels have assigned values of more than two numberpossibilities. The RIP renders the display list into a “contone”(continuous tone) byte map for the page to be printed. This contone bytemap represents each pixel location on the page to be printed by adensity level (typically eight bits, or one byte, for a byte maprendering) for each color to be printed. Black text is generallyrepresented by a full density value (255, for an eight bit rendering)for each pixel within the character. The byte map typically containsmore information than can be used by the printer. Finally, the RIPrasterizes the byte map into a bit map for use by the printer. Half-tonedensities are formed by the application of a halftone “screen” to thebyte map, especially in the case of image objects to be printed.Pre-press adjustments can include the selection of the particularhalftone screens to be applied, for example to adjust the contrast ofthe resulting image.

[0046] Electrographic printers with gray scale printheads are alsoknown, as described in International Publication Number WO 01/89194 A2,incorporated herein by this reference. As described in this publication,the rendering algorithm groups adjacent pixels into sets of adjacentcells, each cell corresponding to a halftone dot of the image to beprinted. The gray tones are printed by increasing the level of exposureof each pixel in the cell, by increasing the duration by way of which acorresponding LED in the printhead is kept on, and by “growing” theexposure into adjacent pixels within the cell.

[0047] Ripping is printer-specific, in that the writing characteristicsof the printer to be used is taken into account in producing the printerbit map. For example, the resolution of the printer both in pixel size(dpi) and contrast resolution (bit depth at the contone byte map) willdetermine the contone byte map. As noted above, the contrast performanceof the printer can be used in pre-press to select the appropriatehalftone screen. RIP rendering therefore incorporates the attributes ofthe printer itself with the image data to be printed.

[0048] The printer specificity in the RIP output may cause problems ifthe RIP output is forwarded to a different electrographic printer. Onesuch problem is that the printed image will turn out to be either darkeror lighter than that which would be printed on the printer for which theoriginal RIP was performed. In some cases the original image data is notavailable for re-processing by another RIP in which tonal adjustmentsfor the new printer may be made.

[0049]FIG. 2 illustrates a schematic block diagram of the function ofrender circuit 39. For exemplary purposes only, it is assumed thatbinary image data is provided by the RIP on line 310 to convertercircuit 312 which, in this example, converts the data from binary tomulti-bit data, such as eight bit data. For example, the pixel value maybe converted from a 1 or 0 value, to a value ranging from 0 to 255 andprovided on a line 314. For simplicity, it will be assumed that thepixel being treated or the pixel in question (PIQ) values on line 314 iseither 0 or 255. The 8 bit PIQ value is provided to an edgedetermination circuit 316 which applies a standard 3×3 edge Laplaciankernel circuit to determine if the PIQ is an edge pixel. The results (A)of this edge determination is provided on a line 317 to mapping circuit318 and a pixel object width determination circuit 320. In otherterminology, circuit 316 flags whether the PIQ is edge pixel or not. Anedge is defined as a transition between background and foreground. Edgepixels define the transition between background and foreground pixels.Background pixels are defined as pixels having relatively little or noprintable or marking information within. Print or marking information isthe digital value assigned to the pixel which results in a certainamount of marking material, such as ink or toner, to be deposited on areceiver, where the amount of material has a functional relationship tothe digital value. For example, in the present embodiment, higherdigital values may mean higher amounts of toner being deposited,resulting in a visually darker pixel. An inverse relationship could alsobe employed, however. Foreground pixels are defined as pixels havingsome printable or marking information within. Foreground pixels may beeither interior pixels, edge pixels, one line pixels, or two linepixels. Interior pixels are foreground pixels that are not edge pixels,one line pixels, or two line pixels.

[0050] The output on line 314 from converter circuit 312 is alsoprovided to a 3×3 directional lookup table circuit 322. Circuit 322assigns a designation or flag to the PIQ depending on the pixelssurrounding it in a 3×3 matrix. With reference to FIG. 4, it can be seenthat there are eight different configurations for an edge PIQ in thecenter of a 3×3 pixel matrix. The PIQ is designated by an “X”. Eachmatrix is assigned an arbitrary designation, and in this example thedesignations are letters indicating direction of the adjoining pixelsfrom the PIQ. Other letters or numerical designations may just as wellbeen assigned. The output (D) of directional lookup table circuit 322 isprovided on a line 323 to character or object pixel width determinationcircuit 320.

[0051] Pixel width determination circuit 320 determines if the edge PIQis part of an object that is one or two pixels wide, and flags the datawith a tag (B) accordingly on a line 321.

[0052] Mapping circuit 318 is provided information from multiple sourcesand provides an output on a line 340 to the writer interface. The inputsto mapping circuit are the edge detection pixel information A on line317, object width information B on a line 321, and original image PIQdata C on line 314. In addition, assignment values for interior pixels,edge pixels, one pixel wide lines and two pixel wide lines are providedto mapping circuit 318 on lines 330, 332, 334 and 336. These assignmentvalues are new values that will be given to the PIQ, depending uponwhether the PIQ is part of a two pixel wide object (2PV), one if the PIQis part of a one pixel wide object (1PV), one if the PIQ is an edgepixel of an object more than two pixels wide (EPV), and another value ifthe PIQ is an interior (not background) pixel (IPV). Background pixels(white area) are not changed by this particular algorithm, althoughanother might do so to achieve a desired effect.

[0053] The types of assignment parameters and the number of assignmentvalues may be determined in an unlimited number of ways. For example,they may be provided by a user in response to a particular effect theprint operator wishes to obtain by programming through a user interface,mechanical switches or other adjustments. The assignment values may alsobe determined automatically by the controller or LCU in response toprinter operational parameters, operator input or other input. Theassignment values and parameters may be combined to determine newassignment parameters. However they may be determined, new pixel tone orexposure values will be assigned to the PIQ post rip. One primary factorin new pixel tone value assignment is the location of the PIQ in theimage in relation to surrounding pixels. Although the input to therendering circuit is explained as a binary input, the input may also bea multi-bit input wherein new multi-bit PIQ exposure values will beassigned for the input PIQ exposure values. Also, it is to be noted thatthe digital input PIQ exposure values may be either binary or multi-bit,meaning the input digital image may be either binary or multi-bit.

[0054] Rendering circuit 39 is an in line interface, or serial interfacein that it is provided between the RIP and the writer interface. Imagerendering can therefore be accomplished independent of the printer orother printer components discussed hereinbefore, such as the RIP orwriter interface. It may be implemented with hardware (such as acomputer or processor board), software, or firmware as those terms areknown to those skilled in the art. The image rendering of the presentinvention can also be accomplished utilizing data from the other printercomponents, such as data typically utilized for process control. Inaddition, image rendering may be set or programmed by an operator orother external or remote source in order to achieve a particular effector effects in the printed output. Implementing a rendering circuit inhardware just prior to gray level writer allows for lower bandwidthrequirements right up to last stage before exposure. The writer may beany grey level exposure system.

[0055] Referring to FIG. 3, a flowchart of a method of image renderingby circuit 39 is provided. In a first step 210, binary image data isreceived from the data source 36, preferably after it has been ripped bythe RIP 37. In a step 212, circuit 39 determines whether the pixel beingtreated or the pixel in question (PIQ) is an edge pixel. Edge pixels ofbinary images may be detected using any of a number of standardalgorithms known in the art (William K. Pratt, Digital Image Processing,Second Edition, John Wiley and Sons, 1991, Chapter 16). The edge can bethe white edge or black edge. The black edge is used for “thinning” orlightening and the “white edge” is used for thickening or darkening. Todetect black edges, the binary image is converted to 8 bits (e.g. 0→0and 1→255) and a standard 3×3 edge Laplacian kernel is applied.Preferred embodiment uses the following kernel: 0 −1 0 −1 4 −1 0 −1 0

[0056] The result of this operation is an image in which all imagepixels are 0 except for edge pixels which have a value of 255. To detectwhite edges, the binary image is converted to 8 bits and inverted (e.g.0→255 and 1→0). White edges of text and other features are detected whenthe image is to be darkened or lines and halftones dots are to be madewider. The white pixel edges are then replaced with a gray level towiden or extend the exposed region. The amount of gray level addeddetermines the degree to which the image is darkened. The particularedge detection algorithm utilized can be combinations and refinements ofstandard algorithms known in the art. In a thinning case, changing theedge pixels of each halftone cell to gray lightens the printed pictorialimage. In a thickening case, adding gray to the white edge pixels aroundthe halftone cell darkens the image.

[0057] If the PIQ is determined not to be an edge pixel, then in a step214, the determination is made whether the PIQ value is zero orsomething other than zero. If the PIQ value is zero, then the PIQ valueis assigned the background pixel value BPV, (which for exemplarypurposes in this case is zero) in a step 215, since it is part of thebackground. If the PIQ value is not zero then it's assumed it's aninterior pixel (solid area pixel) and a new interior pixel value (IPV)is assigned to it in a step 216.

[0058] If the PIQ in step 212 was determined to be an edge pixel, a step217 determines whether the image rendering is in a thinning mode or awidening mode. These modes will be discussed in more detail hereinafter.If a thinning mode is desired, then a determination is made in a step218 as to whether the PIQ is an edge pixel of a line or object that isone pixel wide. If yes, then the PIQ is assigned a new one pixel widevalue (1PV) in a step 220. If the answer to step 218 is no, then adetermination is made in a step 222 whether the PIQ is part of a line orobject that is two pixels wide. If yes, then a two pixel wide value(2PV) is assigned to the PIQ in a step 224.

[0059] If the answer to step 217 is no, then the PIQ is assigned an edgepixel value (EPV) in a step 226.

[0060] It is to be noted that the flowchart of FIG. 3 may be analgorithm that is performed as part of the mapping circuit 318 orfunction as illustrated in FIG. 2. Also, as can be seen in FIG. 2,binary pixel data is provided by the RIP to the input of the imagerendering circuit and multi-bit pixel data is output to the writer.Variations of how the data is converted, and what values are assigned tothe different pixels are limitless and depend on what alterations toprinted images is desired by the user. Also, it can be seen that therendering algorithm begins with or is based on detecting edges or edgepixels.

[0061] Referring to FIG. 4, a binary bitmap of six objects toned orprinted in an array of pixels is illustrated. In each array, the centerpixel is considered the PIQ. Presuming the PIQ is an edge pixel, thereare six different relational configurations or objects defined as towhere the PIQ is located with relation to the surrounding object. Thesix possibilities are provided as a directional look up table (DIR LUT)or directional LUT. Six variable values S, N, E, W, NE, SW, SE, NW areassigned the six configurations. It can be seen that in half the cases,the PIQ is a multi-edge pixel in that it is part of more than one edge.In the present embodiment, all other directional relationalconfigurations would be assigned a value of zero. The PIQ and adjacentobject may be identified using any of a number of edge detectionalgorithms, such as the Laplacian kernel described hereinbefore. Withthis analysis, determination of the orientation of the PIQ with respectto adjacent pixels can be made. Different pixel values may be assignedto the different orientations to achieve different print results.

[0062] The following represents Pseudo code for 1 pixel wide line pixelvalue assignment decisions in accordance with the exemplary algorithmfor block 320 of FIG. 2: If pixel from A is an edge pixel and pixelvalue from DIR LUT is 0, Then pixel is part of 1 pixel wide line.

[0063] The following represents Pseudo code for a 2 pixel wide linepixel value assignment decisions in accordance with exemplary algorithmfor block 320 of FIG. 2: If pixel from A is an edge pixel, Then if pixelfrom DIR LUT is a E and if adjacent pixel to the right is a W     ThenPixel is part of a two pixel wide line   Else if pixel from DIR LUT is aSE and if pixel on next line and to the right is a NW     Then Pixel ispart of a two pixel wide line   Else if pixel from DIR LUT is a S and ifpixel on next line and directly below is a N     Then Pixel is part of atwo pixel wide line   Else if pixel from DIR LUT is a SW and if pixel onnext line and to left is a NE     Then Pixel is part of a two pixel wideline   Else if pixel from DIR LUT is a W and if adjacent pixel to theleft is a E     Then Pixel is part of a two pixel wide line   Else ifpixel from DIR LUT is a NW and if pixel on previous line and to left isa SE     Then Pixel is part of a two pixel wide line   Else if pixelfrom DIR LUT is a N and if pixel on previous line and directly above isS     Then Pixel is part of a two pixel wide line   Else if pixel fromDIR LUT is a NE and if pixel on previous line and to right is a SW    Then Pixel is part of a two pixel wide line   Else pixel is an edgepixel

[0064] As described hereinbefore, the RIP provides image data to arender circuit 39. The RIP 37 and render circuit 39 can be dedicatedhardware, or a software routine such as a printer driver, or somecombination of both, for accomplishing this task.

[0065] The rendering circuit or algorithm of the present inventiondefines, classifies or identifies each pixel as a particular kind ofpixel and reassigns pixel values as a function of their classification,where the different classification reassignment values may beindependent of each other. For example, the algorithm may classify eachpixel as either a background pixel, interior pixel, edge pixel, one linepixel, or two line pixel and reassign new values to these pixelsaccording to those classifications and independent of the each other.For example, interior pixels may be reassigned new values while edgepixel remained unchanged, or edge pixels may be reassigned new valueswhile leaving interior pixels unchanged, or edge pixel values may belowered with respect to interior pixel values, or interior pixel valuesmay be lowered with respect to edge pixel values, etc. It can be seenthere are unlimited variations to the present rendering algorithm. Forexample, the rendering circuit or algorithm of the present invention maydefine each pixel as either a background pixel, interior pixel or edgepixel and reassign these values independently of each other. Many pixelclassifications may be defined, examples of which have been definedherein with the designations background pixel (BP), foreground pixel(FP), interior pixel (IP), edge pixel (EP), one line pixel (1W), twoline pixel (2W), N, S, E, W, NE, NW, SE, SW, Y, Z, etc.

[0066] Referring to FIGS. 6a-6 f, wherein a character is represented ina pixel grid. FIG. 6a is an illustration of a binary bitmap of acharacter. It can be seen that the pixels are either all black (filledwith solid area density of maximum toner d_(max)) or have no toner andhave area toner density of zero.

[0067]FIG. 6b illustrates the toned character of 6 b after assigning alower pixel value to both interior pixels and edge pixels. In otherwords, IPV and EPV were reassigned from d_(max) in FIG. 6a to d_(x),where d_(x) is lower than d_(max).

[0068]FIG. 6c illustrates the edge pixels of the character when thecharacter is undergoing thinning.

[0069]FIG. 6d illustrates assignment of new EPV and IPV values for theedge and interior pixels of the character after thinning has occurred.

[0070]FIG. 6e illustrates the edge pixels of the character when thecharacter is undergoing thickening.

[0071]FIG. 6f illustrates assignment of new EPV and IPV values for theedge and interior pixels of the character after thickening has occurred.

[0072] It can be seen from these figures that after operation of thealgorithm, the edge pixels and interior pixels may be assigned greylevels independently. Once edge pixels are detected, the remainingpixels consist of either “background” pixels (white unprinted area) or“interior” pixels (foreground less edge pixels). Interior pixels can bedistinguished from background pixels in that if a pixel is NOT an edgepixel (from above) and if in the original image data the pixel is a 0(no marking) then the pixel is a background pixel. On the other hand, ifthe pixel is NOT a edge pixel (from above) and if in the original imagedata the pixel is a 1 (marking) then the pixel is an interior pixel.With the rendering circuit, the exposure level of interior pixels can bechanged. Second and subsequent layers of edge pixels can be detected bysimply performing the edge detection algorithm on the interior pixelswhich remain after the edge pixels are removed. Interior pixels wouldthen refer to pixels remaining after all layers of edge pixels have beenremoved. A flow chart for this type of iteration is illustrated in FIG.17.

[0073] The steps taken in FIG. 17 begin with step 610 of receiving imagebitmap data. In a step 612, the edge pixels are identified and assigneda new value EPV₁ in a step 614 and thereafter sent to the writer. TheEdge 1 pixels identified in step 612 are also assigned a value of zeroin a step 616, thereby creating a new “virtual” Edge 2. The Edge 2pixels are identified in a step 618 and reassigned a new pixel valueEPV₂ in a step 620 and thereafter sent to the writer. The Edge 2 pixelsidentified in step 618 are also assigned a value of zero in a step 622,thereby creating a new “virtual” Edge 3. The Edge 3 pixels areidentified in a step 624 and reassigned a new pixel value EPV₃ in a step626 and thereafter sent to the writer. This process can be iterated manytimes over so that Edge N-1 pixels are assigned a value of zero in astep 628, thereby creating a new “virtual” Edge N. The Edge N pixels areidentified in a step 630 and reassigned a new pixel value EPV_(N) in astep 632 and thereafter sent to the writer.

[0074] A process similar to that described above process may be utilizedto thicken or expand the size of an object edges by simply assigning avalue higher than zero, such as one or d_(max) in steps 616, 622, 628,etc. in order to create a new edge, real or virtual.

[0075]FIGS. 5a-5 d illustrate different alterations that may beaccomplished using an iterative edge detection, or edge “peeling”technique.

[0076]FIG. 6b illustrates another technique for altering the image,which is to assign the same grey level to both edge and interior pixels.

[0077] Referring to FIGS. 7a-7 d in conjunction with FIGS. 2 and 3,wherein a single pixel width character or line is represented in a pixelgrid. FIG. 7a is an illustration of a binary bitmap of a one pixel widecharacter. It can be seen that the pixels are either all black (filledwith solid area density of maximum toner d_(max)) or have no toner andhave area toner density of zero. FIG. 7b illustrates assignment of eightbit values for the binary values of FIG. 7a after determination of theedge pixels according to a Laplacian kernel. FIG. 7c illustratesassignment of direction values for the pixels surrounding characterpixels after application of the assignment algorithm described andillustrated hereinbefore in FIG. 4. FIG. 7d illustrates the assignmentbackground pixel, edge pixel and of direction values for the pixel gridin accordance with the algorithm described and illustrated hereinbefore.

[0078] Referring to FIGS. 8a-8 d in conjunction with FIGS. 2 and 3,wherein a two pixel width character or line is represented in a pixelgrid. FIG. 8a is an illustration of a binary bitmap of a two pixel widecharacter. It can be seen that the pixels are either all black (filledwith solid area density of maximum toner d_(max)) or have no toner andhave area toner density of zero. FIG. 8b illustrates assignment of eightbit values for the binary values of FIG. 8a after determination of theedge pixels according to a Laplacian kernel. FIG. 8c illustratesassignment of direction values for the pixels surrounding characterpixels after application of the assignment algorithm described andillustrated hereinbefore in FIG. 4. FIG. 8d illustrates the assignmentbackground pixel, edge pixel and of direction values for the pixel gridin accordance with the algorithm described and illustrated hereinbefore.

[0079] As described, in order to preserve fine lines (avoid loss ofinformation), one and two pixel wide lines are each detected when“thinning”. All pixels that comprise a one or two pixel wide line arecategorized as edge pixels after the laplacian operation. It is to beappreciated that many methods known in the art can be used to identify 1and 2 pixel wide lines. As described herein, to distinguish 1 and 2pixel wide lines from other edge pixels, the original image which hasbeen converted to 8 bits is operated upon by a 3×3 direction look uptable (DIR LUT). The resulting output contains information identifyingthe edge gradient of all edges. Using information from the originalimage, the output of this operation along with edge pixel data from theimage created by the laplacian operation is used to identify pixelswhich are part of a one pixel wide line from pixels which are part of atwo pixel wide line. Since one pixel wide lines can be detected anddistinguished from 2 pixel wide lines, each type of line can have aunique gray level assigned to it which in turn can be different fromother edge pixels.

[0080] Note that if onion skin layering is applied, there may existfirst layer edge pixel values, second layer pixel values, etc. The graylevel range for interior and edge pixels is 0 (no exposure) to 255(maximum exposure). When thinning, one and two pixel wide lines have arange from some minimum exposure (not 0) to the maximum exposure. Thisis so that these lines will appear on the print. However, the presentinvention does not preclude setting gray level on these in order tointentionally erase fine lines.

[0081]FIG. 9 illustrates an example of an interface for an operator toadjust the pixel density assignment values. Other inputs can beutilized. As discussed previously, an operator can adjust theseparameters in different ways to achieve a desired print result. Forexemplary purposes only, there is shown adjustments for the values ofInterior Pixel, Edge Pixel, One Pixel Wide, Two Pixel Wide, TonerConsumption, Character Linewidth, Shadow, Asymetry and ExposureModulation (lightness/darkness). The adjustments can be made utilizing auser interface or mechanical switch connected the printer, theparticular kind and style of interface being variable. Providing a userwith an interface allows that user to make many adjustments to the imageso as to achieve a particular print output without having to rerip theimage. As discussed herein, different printers provide different printcharacteristics. The user interface provides a means to adjust oneprinter to mimic or appear like another printer on the fly, so to speak.That is, adjustments can be made while the printer is operating so thatprint output may be analyzed quickly and iteratively with littleinconvenience. Not all of the adjustments in FIG. 9 would be located inthe same interface, and other adjustments not specifically shown thereinare contemplated.

[0082] The present rendering circuit may be used in any type ofelectrographic system, of any size or capacity in which pixel exposureadjustment value is selected prior to printing. The printer processes abit map of the image to be printed and identifies edge pixels first andthen identifies other types of pixels in that image. The exposure levelfor these pixels is then set by the printer according to new pixelexposure adjustment values according to density adjustments performed bythe printer. Many printed image and object characteristics, parametersand utilities may be affected by this method. For instance, a patternmay be provided to interior pixels. This would be applied in the mappingsection where the interior pixel value is assigned. A benefit to thepresent algorithm is that changes may take effect immediately becauseprocess control controls to the same density.

[0083] When combining output from different printers to create onedocument, it is sometimes desirable to have the look and feel of theprinters to be as similar as possible. Also, bitmaps of images ripped onone printer are sometimes printed on a printer with differentcharacteristics than the original printer for which they were ripped.The present invention provides a method to obtain this result withoutreripping images and without adjusting other machine setup parameters(e.g. electrophotographic process setpoints). Appearance aspects whichmay be adjusted include but are not limited to text, line widths andpictorial tone scale. Feel aspect include but are not limited to tonerstacking (tactile feel of toner stack). Image adjustments made utilizingthe rendering circuit described herein take immediate effect on printoutput and therefore avoids any time delays normally associated withclosed loop control system adjustment to electrophotographic processsetpoints.

[0084] Sometimes users are willing to tradeoff image quality to attainhigher toner yield per printed page. Another aspect of the renderingcircuit is to provide the user with a “knob” or adjustment to adjusttoner consumption at various levels of image quality, as shown in FIG.9. A user is provided the ability to lower certain pixel values, likeinterior and edge pixels, thereby lowering the amount of toner beingdeposited in the affected pixels and thereby lowering overall tonerconsumption. A user can adjust the printed image in this manner so as tominimize toner consumption while maintaining acceptable image qualitywithout having to rerip the image.

[0085] To this end, it can be seen that the rendering circuit accountsfor all pixels of an image to be printed, and determines toner levelsfor each pixel. With this being the case, the printer may track ormonitor total toner consumption of the printer accurately by adding orcalculating the toner deposited for each line, character, and imageprocessed and printed. By counting the number of edge (those having atleast one adjacent pixel non toned) and interior (those having alladjacent pixels toned) and applying different conversion factors (tonerusage per pixel to each), a prediction of toner usage can be achieved.Toner consumption by line, page, job or multiple jobs can beaccomplished. This estimate has customer applications as well aspotential uses in toner replenishment/toner concentration control in theprinter itself. The conversion factors applied can also be dependent onthe density targets used in printers that have variable density controlallowing the customer to select the best cost/quality point for eachjob. As an example, 6% coverage documents made up of text and made up of1 inch solid squares have been shown to consume between 0.0397 and0.0294 grams of toner per sheet respectively. This difference of 33%occurs even though the total number of black pixels for the twodocuments differs by less than 0.5% Analyzing these two images for edgeand interior pixels indicates that edge pixels consume 1.3 times thetoner that interior pixels do. Accounting for the edge and interiorpixels separately clearly yields improved estimates for tonerconsumption than estimates using only pixel counts.

[0086] As mentioned hereinbefore, the process of electrography involvesforming an electrostatic charge image on a dielectric surface, typicallythe surface of a photoconductive recording element that is being drawnor otherwise conveyed through a developing station or toning zone. Theimage is developed by bringing a two-component developer into contactwith the electrostatic image and/or the dielectric surface upon whichthe image is disposed. The developer includes a mixture of pigmentedresinous particles generally referred to as toner andmagnetically-attractable particles generally referred to as carrier. Thenonmagnetic toner particles impinge upon the carrier particles andthereby acquire a triboelectric charge that is opposite the charge ofthe electrostatic image. The developer and the electrostatic image arebrought into contact with each other in the toning zone, wherein thetoner particles are stripped from the carrier particles and attracted tothe image by the relatively strong electrostatic force thereof. Thus,the toner particles are deposited on the image. The magnetic carrierparticles are drawn to the toning shell by the rotating magnets therein.This magnetic force generally does not affect the nonmagnetic tonerparticles.

[0087] However, within the toning zone the toner particles are affectedby forces other than the electrostatic force attracting the toner to theimage and which may degrade image quality. These forces include, forexample, repulsion of toner from the portion of the dielectric surfaceor photoconductive element that corresponds to the background area ofthe image, electrical attraction of the toner particles to the carrierparticles, repulsion of toner particles from other toner particles, andelectrical attraction to or repulsion from the toning shell depending onthe polarity of the film voltage in the developer nip area. There arecertain methods of compensating for and/or balancing the effect of theseother forces on the nonmagnetic toner particles to prevent anysignificant adverse effect on image quality. However, the forces ontoner particles having magnetic content are very different from theforces on nonmagnetic toner.

[0088] In addition to the electrical forces acting on nonmagnetic toneras described above, toner having magnetic content is subjected tomagnetic forces, such as, for example, magnetic attraction of the tonerparticles to the carrier particles, to other toner particles, and to therotating core magnet. All of these magnetic forces are generally in adirection away from the film or electrostatic image carrier. The onlyforce acting to draw the toner onto the electrostatic image carried bythe film or dielectric carrier is the electric force. Thus, the magneticforces tend to counteract the electric attraction of toner particles tothe image. The strength of the electric force relative to the magneticforces becomes stronger as the distance between the image and the coremagnet increases. Therefore, the toner tends to be deposited on thetrailing edge of the film or dielectric carrier. The result is an imagehaving solids with heavy toning on the trailing edge of the image, andcross track lines (i.e., lines perpendicular to the direction of travelof the dielectric support member or film) that are wider than thecorresponding in track lines (i.e., lines that are parallel to thedirection of travel of the dielectric support member or film).

[0089] This “Fringe” field effect (the condition wherein fringeelectromagnetic fields around the edges of lines on the photoconductorresult in toner build up at edges of lines on the printed material) canbe a problem for some printers. The rendering circuit described hereinprovides a method to reduce the toner build up on the edges by adjustingthe IPV, EPV, 1PV or 2PV parameters accordingly to reduce or counteractthese effects. For example, FIGS. 5a-5 d illustrate a character havingdifferent exposure values assigned to different layers which may beutilized to minimize the fringe field effect on image quality.

[0090] As described hereinbefore, d_(max) control uses the signal from atransmission densitometer circuit reading a d_(max) patch to adjust V₀and/or E₀ electrophotographic parameters concurrently to maintain solidarea density. In addition to d_(max) a shadow detail patch may bewritten using approximately 70-90% pixel pattern or at 70-90% of d_(max)exposure in a flat field pattern at the selected edge and interior pixelexposure values determined by the rendering circuit during tuning priorto the run. Based on the densitometer signal generated by this patch,the edge and interior pixel exposure values may be adjusted to maintainthe desired shadow detail density (or large line character width) byadjusting or reassigning pixel values. In addition, a highlight detailpatch may be written using approximately 5-20% of d_(max) exposure blackpixels in a flat field pattern using the selected edge, interior, andsmall feature pixel exposure values determined by the rendering circuitduring tuning prior to the run. Based on the densitometer signalgenerated by this patch, the small feature pixel values may be adjustedto maintain the desired highlight detail density (or fine line characterlinewidth) by reassigning one or more of EPV, 1PV and 2PV.

[0091] As described herein, it is possible using the rendering circuitto apply reduced exposure at all edges of characters, but this may leadto too large a reduction in line width since the minimum adjustment isapplied to two pixels. This is especially true of characters printed ina small font size. To achieve less linewidth reduction, half ofcharacter edges may be reduced (top and left edges only for example).This may lead, however, to an apparent shift of the center of thecharacters locations and this may be undesirable for a particularapplication (for instance with kerned fonts and small font sizecharacters). To achieve linewidth reductions less than those achievedwith all edge pixel exposure reductions, and avoid apparent centershifts of small font size characters caused by top/left or bottom/rightedge exposure reductions, the rendering circuit may apply an alternativealgorithm and assign pixel values such that closed characters (thosehaving enclosed spaces such as o, d, b, etc.) have reduced exposure onlyfor the interior or exterior edges of enclosed areas. For example, FIG.10 illustrates a letter “O”, (which is a closed character), havinginterior edges and exterior edges with different exposure valuesassigned to them. This helps to maintain the center location ofcharacter without achieving excessive linewidth reduction. Remainingstraight portions of the characters may have only one edge exposurereduced. A similar algorithm may be applied to characters havingpartially enclosed spaces (such as v, c, m, n etc.) whereby only theinterior or exterior edge is exposure modified. Characters with multiplepartially enclosed spaces (such as t, y, w, m, etc.) would require alarger set of rules to avoid modifying both edges of any strokes, but itshould be possible to generate a consistent set of rules capable ofavoiding such conflicts.

[0092] Desired edge exposure reductions may utilize a two dimensionaloperator of sufficient size to completely enclose the largest sizecharacter to which it will be applied. If an area is identified in theoperator field of view as a separate object, it may then operate on theobject in accordance with the rendering algorithm described herein toreduce apparent linewidth while minimizing the apparent center shiftingof characters.

[0093] As the interior pixel (solid area density) exposures drop belowcertain levels, electrophotographic process nonuniformities becomeapparent in the solid area imaging. Assigning a pattern of differentexposure values for interior pixels (multiple IPVs rather than using asingle exposure for all interior pixels) reduces the visibility of EPprocess non-uniformity. The particular pattern used is analogous to ahalftoning pattern for binary imaging, except the modulation is betweendifferent non-white exposure levels. The pattern of differing densitypixels tends to obscure streaks and bands that become visible in flatfields of same level exposure pixels and minimizes the visibility ofnon-uniform density. The nonuniformities can be identified or measuredin a number of ways, examples of which are visually inspecting theprinted output or utilizing a density patch and measuring densitythereof. The pattern can be of any size with any number of differentexposure values such that it creates the desired average interior pixeldensity when printed to reduce print nonuniformities.

[0094] In this regard, the present invention is useful when printingmagnetic toner or ink. Magnetic Ink Character Recognition (MICR)technologies have been used for many years for the automated reading andsorting of checks and negotiable payment instruments, as well as forother documents in need of high speed reading and sorting. As well knownin the art, MICR documents are printed with characters in a special font(e.g., the E13-B MICR font in the United States, and the CMC-7 MICRstandard in some other countries). Typically, MICR characters are usedto indicate the payor financial institution, payor account number, andinstrument number, on the payment instrument. In addition to the specialfont, MICR characters are printed with special inks or toners thatinclude magnetizable substances, such as iron oxide, that are magnetizedfor facilitating an automatic reading process by a reading instrumentwhich is sensitive to the magnetic fields surrounding the printed MICRcharacters. The magnetized MICR characters present a magnetic signal ofadequate readable strength to the reading and sorting equipment, tofacilitate automated routing and clearing functions in the presentationand payment of these instruments.

[0095] The relatively heavy loading of iron oxide in conventional MICRtoner for electrographic MICR printing has been observed to adverselyaffect the image quality of the printed characters, however. It isdifficult to achieve and maintain an adequate dispersion of the heavyiron oxide particles in the toner resin. In addition, the toning andfusing efficiencies of MICR toners are poorer than normal (i.e.,non-MICR) toners, because of the magnetic loadings present in the MICRtoner. Accordingly, the image quality provided by MICR toner is may bepoorer than those formed by normal toner, unless the printing machinemakes adjustments to compensate. The present rendering circuit providesa way to adjust MICR toner density in parts of characters so as tominimize the printing nonuniformities resultant therefrom. By varyingpixel toner density values as a function of pixel character location asillustrated in the exemplary drawings herein, the concentration ofmagnetic toner particles may be adjusted to improve the readability ofthe printed characters by reading instrumentation.

[0096]FIG. 11 illustrates an example of a typical tone reproductioncurve, also referred to in the art as a “gamma” curve, illustrating thetypical performance of conventional printers in reproducing tonedensity, in this example for gray scale printing. In this plot, thehorizontal axis corresponds to input intensity between white (nointensity) and black (full intensity); the vertical axis corresponds tothe corresponding printer output density, on the hard copy medium,between d₀ (no density) and d_(max) (full density). Ideally, thetransfer function from input intensity to output density would be a 45°line, shown as ideal plot I in FIG. 11, along which the output densityexactly matches the input intensity.

[0097] Printer performance follows a non-linear “S-shaped” tonereproduction curve, for example as shown by actual plot A in FIG. 11,often referred to as the “gamma” curve. Along this tone reproductioncurve, output density is generally less than that specified by low inputintensity values (i.e., below the ideal I); this portion of the tonereproduction curve is referred to as the “toe”, shown by region T inFIG. 11. The output densities in the “toe” region are also referred toas “highlight” densities. At the other extreme, for high input intensityvalues, output density is generally higher than that specified by theinput (i.e., above the ideal I). These output densities in the“shoulder” region of the tone reproduction curve, for example in regionS of plot A in FIG. 11, are also referred as “shadow” densities. Forboth the highlight and shadow densities, the inaccuracy in tonereproduction is generally manifest by inaccuracies in the printedcontrast; the underdensity in highlight regions shows up as washed outregions of the image, while the overdensity in shadow regions shows upby the absence of bright features (loss of detail in dark regions). Inthe “midtone” region of the tone reproduction curve, shown by region MTof plot A in FIG. 11, the error between output density and inputintensity is relatively small, so that midtones produced by the printerclosely match the input signal.

[0098] In many cases, the Raster Image Processor (RIP) described above,by way of which a page description is converted into a bit map outputfor printing by a specific printer of the electrographic or other type,applies gamma correction in this processing. This gamma correctioncompensates for the non-ideal density output of the printer, in effectapplying a transfer function that is the opposite of the tonereproduction curve for the printer (e.g., plot A of FIG. 11). Thiscorrection will generally be implemented by increasing the densityoutput for lower input intensity values, and decreasing the densityoutput for higher input intensity values. To at least a firstapproximation, the correction amounts to the selection of a gamma value,which is a compensating factor corresponding to the degree of curvatureof the actual tone reproduction curve A from the ideal I. As notedabove, the actual correction may be carried out by selection of theappropriate halftone screens using higher density halftone screens forhighlight densities, and lower density halftone screens for shoulderdensities.

[0099] According to conventional approaches, the selection of theappropriate halftone screens for a given printer or printer typerequires a trial and error process. The correct d_(max) output densitylevel must first be correlated to full density input. Once d_(max) isset, then a representative image is processed using a trial set ofcorrections for highlight and shadow densities; after analysis of theoutput image, the corrections may be adjusted and the image processedagain. Upon convergence to the desired output, additional images may beadjusted using the corrections (e.g., the selected set of halftonescreens) determined in the trial and error process, and printing cancommence. To the extent that the iterative setting of shoulder and toecorrections must be performed for a given printer, or on specificimages, this procedure is time consuming and costly.

[0100] Because of printer specificity in the RIP process, RIP output forone printer or printer type cannot be forwarded to a differentelectrographic printer without risking that the printed image will haveincorrect gamma correction for the images. In other words, the gammacorrection in the RIP output based on the printer for which the originalRIP was performed will likely not correspond to the tone reproductioncurve of a different printer.

[0101] As discussed above, U.S. Pat. No. 6,121,986 provides a solid areadensity control system, in which the optical density of maximum densitypatches, and of less than maximum density patches, is controlled inresponse to the measured performance of the electrographic printer. Thissolid area density control adjusts the output density d_(max) duringsetup and operation of the printer, and also can control the outputdensity at different less-than-maximum levels. However, thisconventional solid area density control only controls the solid areaoutput density value d_(max), and cannot separately control highlightand shadow densities. In other words, an increase in solid area outputdensity d_(max) compensates for the underdensity of highlights, butovercompensates for shadows. Conversely, a decrease in d_(max)compensates for the overdensity of shadows, but undercompensates forhighlights. While solid area control approaches stabilize the opticaldensity of the exposed areas, they don't necessarily introducevariations into character linewidths of text (and analogously into thelinewidths of small isolated image features). Linewidth variations aredue in part to fringe field effects. As known in the art, the amount oftoner applied to a pixel on the photoconductor of an electrographicprinter depends upon the difference between the exposure voltage (asapplied by the LED or laser to the photoconductor) and the bias voltageat the toning station; changes in either of these voltages will changethe amount of toner received by the pixel. Fringe effects occur becausethe electric field at the edge of an exposed patch (i.e., those edges ofexposed pixels that are adjacent to unexposed pixels) is much greaterthan the field at the center of the exposed region. It has been observedthat the difference in field magnitude between the edge and the centermay be as high as 3× to 5×. As a result, toner tends to pile up at theedge of an exposed patch of pixels, and at the edge of single exposedpixels surrounded by unexposed pixels. In the case of single pixels,this piling effect can result in single pixel sizes of on the order of90μ in 600 dpi printers that have a theoretical pixel pitch of 42μ.Again, these fringe effects affect both gray scale images and alsofull-black text and make it difficult to adjust image quality to theextent necessary to compensate for differences in characteristicsbetween an electrographic printer for which the image was originallyRIPped, and a different electrographic printer upon which the image isto be printed. These fringe effects are reduced utilizing the renderingcircuit of the present invention by reassigning edge pixels to havelower exposure values (EPV) at the edge of an exposed patch of pixels,and at the edge of single exposed pixels surrounded by unexposed pixels.

[0102] Digitized halftone images processed at different effective screenfrequencies (the number of lines per inch or lpi) often have differentcontrast (appearances) because of differing dot gains depending on theratio of edge and interior pixels as the area coverage changes. FIG. 12illustrates seventeen halftone steps (the percentage of white in eachstep) for three different screen frequencies, 106 lpi, 85 lpi and 71lpi. The relationship of percent lightness to percent black pixels foreach step for each screen frequency is shown in FIG. 13. It can be seenthat the three curves at standard exposure are different, therebyillustrating different halftone images for different screen frequencies.FIG. 14 illustrates a series of lines that are 1, 2, 3, 4, and 8 pixelswide, respectively. FIG. 15 illustrates a graph of linewidth vs thenumber of pixels counted across the line, where white spaces areassigned negative numbers for a particular set of lines with the samelinewidth (for example 8 pixel wide lines). A best fit line 500 can bedrawn through the data points collected. FIG. 16 illustrates a series ofbest fit lines extracted for linewidth vs the number of pixels derivedby selecting a fixed IPV and varying EPV, 2PV and 1PV for eightdifferent cases. It can be seen that there are eight different best fitlines. It can also be seen that there is one particular best fit linethat passes through the zero intercept. The EPV, 2PV and 1PV values forthe zero intercept line was noted and a series of lines similar to thoseshown in FIG. 14 were printed at screen frequencies of 106 lpi, 85 lpiand 71 lpi. The relationship of percent lightness to percent blackpixels for the three screen frequencies were plotted and are shown inFIG. 13, wherein the resulting curves identified as the zero interceptgroup curves. It can be seen that using the EPV, 2PV and 1PV values forthe zero intercept line results in digitized halftone images that arethe same for differing screen frequencies. By using EPV, 2PV and 1PVexposures that are different from IPV exposure, it is possible toachieve linear behavior between character linewidth and the number ofpixels printed that has an intercept of zero. Because the IPV exposurehasn't changed, it is possible to retain good solid area fill byoverlapping interior pixels. Because the relationship between pixelwidth and measured width has a zero intercept, image density forhalftone patterns is not dependent on the ratio of edge and interiorpixels, which means that is it also independent of screen frequency.Using a user interface, the user is therefore able to adjust the solidarea maximum density (IPV) and then select edge pixel exposures (EPV,2PV, 1PV) to achieve a zero intercept of the character linewidth vnumber of pixels curve to minimize screen frequency sensitivity. To thisend, sensitivity to screens having different dot shapes (e.g. round,elliptical, diamond, etc.) may be minimized also.

[0103] While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

What is claimed is:
 1. An electrostatographic recording method forprinting an image on a receiver comprising the steps of: operating aprimary charger to establish a uniform primary voltage level on an imagerecording member; developing a control patch on the image recordingmember, measuring density of the control patch to thereby provide adensity measurement signal; converting the image into a digital bitmapcomprised of an array of pixels wherein each pixel is assigned a digitalvalue representing marking information rendering the digital bitmap bydefining each pixel as either a background pixel, interior pixel, or anedge pixel and reassigning the digital value of one or more of edgepixels or interior pixels independently and as a function of the densitymeasurement signal; modulating electrostatic charge on the imagerecording member as a function of the digital bitmap after rendering. 2.A method in accordance with claim 1, wherein the digital image is abinary image.
 3. A method in accordance with claim 1, wherein thedigital image is a multi-bit image.
 4. A method in accordance with claim1, wherein the reassigning step comprises increasing the value of edgepixels with respect to interior pixels.
 5. A method in accordance withclaim 1, wherein the reassigning step comprises decreasing the value ofedge pixels with respect to interior pixels.
 6. A method in accordancewith claim 1, further comprising performing the defining and reassigningsteps two or more times.
 7. An electrostatographic recording method forprinting an image on a receiver comprising the steps of: operating aprimary charger to establish a uniform primary voltage level on an imagerecording member; developing a control patch on the image recordingmember, measuring density of the control patch to thereby provide adensity measurement signal; converting the image into a digital bitmapcomprised of an array of pixels wherein each pixel is assigned a digitalvalue representing marking information; rendering the digital bitmap bydefining each pixel as either a background pixel, interior pixel, edgepixel, one line pixel, or two line pixel and reassigning the digitalvalue of one or more interior pixels, edge pixels, one line pixels, ortwo line pixels independently and as a function of the densitymeasurement signal; and, modulating electrostatic charge on the imagerecording member as a function of the digital bitmap after rendering. 8.A method in accordance with claim 7, wherein the digital image is abinary image.
 9. A method in accordance with claim 7, wherein thedigital image is a multi-bit image.
 10. A method in accordance withclaim 7, wherein the reassigning step comprises increasing the value ofedge pixels with respect to interior pixels.
 11. A method in accordancewith claim 7, wherein the reassigning step comprises decreasing thevalue of edge pixels with respect to interior pixels.
 12. A method inaccordance with claim 7, further comprising performing the defining andreassigning steps two or more times.
 13. An electrostatographic printingapparatus for printing an image on a receiver comprising: a primarycharger to establish a uniform primary voltage level on an imagerecording member; a developer for developing a control patch on theimage recording member, a densitometer for measuring density of thecontrol patch to thereby provide a density measurement signal; a rasterimage processor for converting the image into a digital bitmap comprisedof an array of pixels wherein each pixel is assigned a digital valuerepresenting marking information a rendering circuit for rendering thedigital bitmap by defining each pixel as either a background pixel,interior pixel, or an edge pixel and reassigning the digital value ofone or more of edge pixels or interior pixels independently and as afunction of the density measurement signal; wherein the developermodulates electrostatic charge on the image recording member as afunction of the digital bitmap after rendering.
 14. An apparatus inaccordance with claim 13, wherein the digital image is a binary image.15. An apparatus in accordance with claim 13, wherein the digital imageis a multi-bit image.
 16. An apparatus in accordance with claim 13,wherein reassigning comprises increasing the value of edge pixels withrespect to interior pixels.
 17. An apparatus in accordance with claim13, wherein reassigning step comprises decreasing the value of edgepixels with respect to interior pixels.
 18. An apparatus in accordancewith claim 13, wherein the rendering circuit further comprisesperforming the defining and reassigning steps two or more times.
 19. Anelectrostatographic printing apparatus for printing an image on areceiver comprising: a primary charger to establish a uniform primaryvoltage level on an image recording member; a developer for developing acontrol patch on the image recording member, a densitometer formeasuring density of the control patch to thereby provide a densitymeasurement signal; a raster image processor for converting the imageinto a digital bitmap comprised of an array of pixels wherein each pixelis assigned a digital value representing marking information a renderingcircuit for rendering the digital bitmap by defining each pixel aseither a background pixel, interior pixel, edge pixel, one line pixel,or two line pixel and reassigning the digital value of one or more ofedge pixels or interior pixels independently and as a function of thedensity measurement signal; wherein the developer modulateselectrostatic charge on the image recording member as a function of thedigital bitmap after rendering.
 20. An apparatus in accordance withclaim 19, wherein the digital image is a binary image.
 21. An apparatusin accordance with claim 19, wherein the digital image is a multi-bitimage.
 22. An apparatus in accordance with claim 19, wherein reassigningcomprises increasing the value of edge pixels with respect to interiorpixels.
 23. An apparatus in accordance with claim 19, whereinreassigning step comprises decreasing the value of edge pixels withrespect to interior pixels.
 24. An apparatus in accordance with claim19, wherein the rendering circuit further comprises performing thedefining and reassigning steps two or more times.